**Core Concept**
Facultative anaerobes are bacteria that can grow and thrive in both the presence and absence of oxygen. They can switch between aerobic respiration and anaerobic fermentation depending on the availability of oxygen in their environment. This adaptability allows them to survive in a wide range of ecological niches.

**Why the Correct Answer is Right**
Facultative anaerobes typically possess mechanisms to switch between aerobic and anaerobic metabolism. They can use both oxygen-dependent and oxygen-independent electron transport chains to generate energy. This flexibility enables them to thrive in environments with varying oxygen levels. In the case of E. coli, it uses a variety of metabolic pathways, including the citric acid cycle and glycolysis, to generate energy in both aerobic and anaerobic conditions.

**Why Each Wrong Option is Incorrect**
**Option A:**
*This option is incorrect because Staphylococcus aureus is typically a facultative anaerobe but it is not the correct answer based on the available options.*

**Option B:**
*This option is incorrect because Pseudomonas aeruginosa is an obligate aerobic, meaning it requires oxygen to grow and cannot survive in anaerobic conditions.*

**Option C:**
*This option is incorrect because Clostridium difficile is an obligate anaerobe, requiring an oxygen-free environment to grow and thrive.*

**Clinical Pearl / High-Yield Fact**
Facultative anaerobes are often found in the human gut and can cause a range of infections, from mild diarrhea to life-threatening sepsis. Understanding their metabolic adaptability is crucial for effective antimicrobial therapy and infection control.

**Correct Answer: D. E. coli**